You can perform initial configuration of OpenLDAP through the following environment variables:

SLAPD_PASSWORD
TLS_REQCERT (default: never)
SLAPD_ORG (default: nodomain)
SLAPD_DOMAIN (default: nodomain)
SLAPD_DC (default: dc=nodomain)
SLAPD_BACKEND (default: MDB)
SLAPD_ALLOW_V2 (default: false)
SLAPD_PURGE_DB (default: false)
SLAPD_MOVE_OLD_DB (default: true)
SLAPD_BINDUSER (default: binduser)
SLAPD_BINDPWD (default: bindpassword)
SLAPD_BINDGROUP (default: bindgroup)

Setting SLAPD_PASSWORD to configure the admin password is required. After the initial setup, a flag file, /etc/ldap/docker-configured, will be created containing the current timestamp, and no further attempts to reconfigure slapd will be made as long as that file exists.